In the field of medical diagnosis and graphic arts, there have been concerns in processing of photographic film with respect to effluent produced from wet-processing of image forming materials, and recently, reduction of the processing effluent is strongly demanded in terms of environmental protection and space saving.
As a result, techniques have been sought which relate to photothermographic materials which can be effectively exposed employing laser imagers and laser image setters and can form clear black-and-white images with high resolution.
Such techniques are described in, for example, U.S. Pat. Nos. 3,152,904 and 3,487,075, both by D. Morgan and B. Shely, or D. H. Klosterboer et al., “Dry Silver Photographic Materials”, (Handbook of Imaging Materials, Marcel Dekker, Inc. page 48, 1991). Also known are silver salt photothermographic materials which comprise a support having thereon an organic silver salt, photosensitive silver halide and a reducing agent. Since any liquid-type processing chemicals are not employed for the silver salt photothermographic materials, they exhibit advantages in that it is possible to provide a simpler environmentally friendly system to customers.
These silver salt photothermographic materials are characterized in that photosensitive silver halide grains as a photo-sensor and an organic silver salt as a supply source of silver ions are incorporated in a photosensitive layer and are thermally developed, commonly at 80 to 140° C., utilizing the incorporated reducing agent, without being fixed.
However, the aforesaid silver salt photothermographic materials tend to result in fogging during storage prior to thermal development, due to incorporation of organic silver salts, photosensitive silver halide grains and reducing agents. Further, after exposure, thermal development is commonly carried out at 80 to 250° C. followed by no fixing. Therefore, since all or some of the silver halide, organic silver salts, and reducing agents remain after thermal development, problems occur in which, during extended storage, image quality such as silver image tone tends to vary-due to formation of metallic silver by heat as well as light.
Techniques which overcome these problems are disclosed in Patent Documents Nos. 1, 2, U.S. Pat. No. 5,714,311, European Patent No. 1096310, and references cited therein, for example, JP-A Nos. 6-208192, 8-267934, 7-2781 and 6-208193. These techniques disclosed therein exhibit some effects, but are not sufficient to meet the market's requirements.
In addition, for the purpose of enhancing covering power(CP), when the number of photosensitive silver halide grains is increased while decreasing the diameter of the aforesaid grains, it has been found that problems occur in which variation and degradation of image quality such as tone of silver images are further accelerated due to effects of light incident to the aforesaid photosensitive slier halide grains during storage of the aforesaid photosensitive silver halide grains after development as well as while viewing them.
A technology employing a leuco dye capable of producing color is disclosed. This technology enables to adjust a hue of silver to a preferred color. The hue of silver is caused by a morphology of silver. Examples of such technology are disclosed in Japanese Patent Publication Open to Public Inspection (hereafter it is referred to as JP-A) Nos. 50-36110, 59-206831, 5-204087, 11-231460, 20002-169249 and 2002-236334. However, this technology is not fully effective to prevent change of color of silver after long-term storage.
It is disclosed another technology to prevent change and deterioration of silver caused by irradiation of light. That technology employs a halogenated compound capable of oxidizing a silver image by irradiation of light. Examples of compounds are shown in Patent Documents-Nos. 3, 4 and JP-A 50-120328. However, these compounds generally tend to exhibit an oxidizing property by an effect of heat. As a result, they have an effect of preventing fog formation but at the same time they may prevent formation of a silver image resulting in a loss of photographic speed, a loss of Dmax and a loss of a silver covering power.
These photothermographic materials can be prepared by performing coating and drying with relatively low energy so that it is popular to coat and dry a coating solution containing solvents such as an organic solvent but drying at a further lower energy is desired for the environment. However, a photothermographic material prepared at a lower temperature tends to result in an increase of the residual solvent content, not only producing problems in photographic performance such as sensitivity or fogging and image lasting quality but also causing odor unsuitable for product quality.
Further, to enhance transportability in equipments, it is necessary to enhance the friction factor of the surface of a photothermographic material.
Accordingly, there has been desired development of a technique for achieving markedly enhanced image quality to overcome problems inherent to the photothermographic imaging material and disadvantages of conventional corresponding techniques.